Controlling resistive switching behavior in the solution processed SiO2-x device by the insertion of TiO2 nanoparticles

The resistive switching behavior of the solution processed SiOx device was investigated by inserting TiO2 nanoparticles (NPs). Compared to the pristine SiOx device, the TiO2 NPs inserted SiOx (SiOx@TiO2 NPs) device achieves outstanding switching characteristics, namely a higher ratio of SET/RESET, lower operating voltages, improved cycle-to-cycle variability, faster switching speed, and multiple-RESET states. Density functional theory calculation (DFT) and circuit breaker simulation (CB) were used to detail the origin of the outstanding switching characteristic of the SiOx@TiO2 NPs. The improvement in resistive switching is mainly based on the difference in formation/rupture of the conductive path in the SiO2 and SiO2@TiO2 NPs devices. In particular, the reduction of resistance and lower switching voltage of TiO2 NPs control the formation and rupture of the conductive path to achieve more abrupt switching between SET/RESET with higher on/off ratio. This method of combined DFT calculation and CB offers a promising approach for high-performance non-volatile memory applications.


Results and discussion
Structure of SiO x based devices. Figure 1a schematically illustrates the cross-bar array architectures of the SiO x and SiO x @TiO 2 NPs resistive switching devices. And, to observe the cross-sectional information of both devices, TEM is measured as shown in Fig. 1b. The TEM image of the SiO x device shows that the ITO/ SiO x /ITO structure is sequentially stacked, and the interface of TE/SiO x is clearly formed. In contrast, the SiO x @ TiO 2 NPs device shows the slightly rough interface of TE/SiO x @TiO 2 NPs, which is related to the insertion of TiO 2 NPs profoundly affecting the roughness of SiO x @TiO 2 NPs. To examine the composition of the SiO x and SiO x @TiO 2 NPs device structures, ToF-SIMS was measured from top to bottom electrode during O ion sputtering with 2 keV. Figure 1c shows that the spectra of ToF-SIMS can be divided into three regions; the first region is only ITO (top electrode), the second region is the SiO x or SiO x @TiO 2 NPs layer, and the last region is ITO (bottom electrode). For the SiO x switching device, Si + is increased in the second region, while In + and Sn + are drastically decreased. O + is continuously detected in all regions, because oxygen is included in all layers. Ti + is not detected in the first and second regions. In the third region, Ti + is found due to the glass substrate, thus it can be negligible 32 . In the case of the SiO x @TiO 2 NPs device, the behaviors of Si + , In + , Sn + , and O + are almost similar to those of the SiO x device. However, a considerable quantity of Ti + is detected in the second region, and we can recognize that the TiO 2 NPs is well inserted into the SiO x matrix.   Fig. 2b and c 26,33 . The prepared SiO x and SiO x @TiO 2 NPs films have a lot more O2 and O3 states than does the conventional SiO 2 film. Generally, this is related to the solution processed SiO x obtaining a large amount of defect states, such as oxygen vacancies or OH groups, which affects the stoichiometry of the SiO x (x < 2) 26,28 . In the Si 2p spectra, the regular SiO 2 (Si 4+ ) and oxygen deficient SiO 2-x (Si 3+ ) are indicated, as shown in Fig. 2b and c 34,35 . The prepared SiO x films synthesized by solution process show higher composition of oxygen vacancies than the SiO 2 synthesized by vacuum process (thermal oxidation or chemical vapor deposition). In general, the amount of oxygen vacancies is expected to change due to the difference in bond dissociation energy in the TiO 2 NPs inserted SiO x system 27,36 . However, in our system, the chemical bonding states are almost similar, due to the small amount of TiO 2 NPs in SiO x matrix. Therefore, the change of the chemical bonding states of SiO x is imperceptible.
Resistive switching characteristics. Figure 3 shows the resistive switching performance of the SiO x and SiO x @TiO 2 NPs devices. First, the voltage is swept to transit from the pristine state to LRS with the compliance current of 0.1 mA. Both devices show bipolar resistive switching behavior with SET process, which is obtained by sweeping the negative voltage, while the RESET process is obtained by sweeping the positive voltage. These   www.nature.com/scientificreports/ processes can be reversibly changed by controlling the voltage. In detail, the SiO x device switches from HRS to LRS (SET process) at − 1.7 V, while the device switches continuously from LRS to HRS (RESET process) by applying voltage to 2.0 V. For the SiO x @TiO 2 NPs device, the SET process occurs at − 1.1 V, which is smaller than the SET voltage of the SiO x device. In addition, different from the SiO x device, the RESET process is obtained twice over; the first RESET process appears at 0.7 V, then the second RESET process with slight resistance change is obtained while increasing the voltage to 2.0 V. The switching speed is obtained that the RSs change under  The SiO x device shows the expectation of the lifetime almost 10 3 s, otherwise the SiO x @TiO 2 NPs device is expected to the lifetime above 10 4 s. To examine the endurance performance, the SET/RESET cycling test was conducted for 10 2 times, and the current level was recorded at 0.1 V. Both devices show a stable LRS/HRS ratio for 10 2 cycles. In particular, the SiO x @TiO 2 NPs exhibits the multiple RSs for 10 2 cycles. Moreover, the endurance of SiO x @ TiO 2 NPs is evaluated for 10 4 cycles under the pulse width of 200 ns. The device shows the stable operation during 10 4 cycles. Long retention time and stable endurance indicate the high reliability of the solution processed SiO x -based resistive switching devices. The distribution of SET and RESET voltages are measured for 10 2 times, and the current level was recorded at 0.1 V for examining the cycle-to-cycle variability of devices. Both devices show a stable LRS/HRS ratio for 10 2 cycles. Especially, the SiO x @TiO 2 NPs exhibits the multiple RSs for 10 2 cycles, and the stable resistive switching occurs in the SiO x @TiO 2 NPs device, compared to SiO x device. The stable endurance, retention, cycle-to-cycle variability performance indicate the high reliability of the solution processed SiO x -based resistive switching devices. From the resistive switching characteristics, it is concluded that the SiO x @TiO 2 NPs device can be expected to high performance and low-power non-volatile memory due to a lower operation voltage, higher on/off ratio, and fast switching speed 23 . Moreover, due to multi-level switching, the SiO x @TiO 2 NPs device is also applicable to multi-level memory.

Resistive switching mechanism.
To discover the origin of the enhancement of performance in the SiO x @TiO 2 NPs device, we clarified the resistive switching mechanism of the SiO x and SiO x @TiO 2 NPs devices. The I-V curves are re-plotted as log I-log V, as shown in Fig. 4a and b. In the SET process of the SiO x resistive switching device, the I-V curve of the HRS exhibits trap-controlled space charge limited current (SCLC) conduction, which is composed of three parts: the Ohmic region (I ∝ V), the Child's law region (I ∝ V 2 ), and the steeply increasing region (I ∝ V n , n > 2) 37 . The oxygen vacancies in the SiO x matrix serve as an electron trap, and form the conductive filament. Thus, the migration of oxygen vacancies is an important role in the deviation of slopes. In the high-voltage region, all traps are filled with electrons, and excessive electrons flow through the conduction band of SiO x (achievement of the SET process). The I-V curve of the LRS shows a linear Ohmic behavior with a slope of 1.07. Likewise, the RESET process is also in good agreement with the trap-controlled SCLC mechanism in HRS. In the SiO x @TiO 2 NPs, the resistive switching mechanism is similar to that of the SiO x  www.nature.com/scientificreports/ for each charging state of oxygen vacancies in SiO 2 and TiO 2 , respectively. In addition, in the case of the VO in SiO 2 , the + 2 is the stable charging state for the energy range (0.0-3.2) eV, 0 is stable for the range (3.2-6.6) eV, and − 2 becomes stable above 6.6 eV. For the TiO 2 , the + 2 charging state is found to be the most stable state over the entire range of bandgap. Considering the Fermi level of SiO 2 and TiO 2 , which is measured in the valance band spectrum of XPS, the most stable charging states of VO are 0 and + 2 for SiO 2 and TiO 2 , respectively. Since the formation energy of VO in TiO 2 is smaller than that of the VO in SiO 2 , VO is more easily generated in TiO 2 than in SiO 2 . Moreover, since the e-field induced migration of VO can occur for charged states, VO in TiO 2 that has 2 + charging states can migrate with smaller e-field. Figure 5c and d show the schematic energy band diagrams of the SiO x and SiO x @TiO 2 NPs devices from the results of energy levels for stable charging states. Band alignments are estimated based on the valance band spectrum in XPS. For both the SiO 2 and TiO 2 systems, VO generates defect states in the bandgap, thus the resistance change of the RERAM device can occur through the generation (SET) and curing (RESET) of VO in oxide. However, considering the band alignment, the activation energy from the Fermi level to defect states is smaller in TiO 2 than in SiO 2 . In addition, the barrier for carrier injection from ITO electrode is lower in TiO 2 with VO than in SiO 2 with VO. Therefore, the resistance of LRS in TiO 2 is lower than the resistance of LRS in SiO 2 .

CB simulations.
To understand the formation/rupture of the conductive path based on the oxygen vacancies for the SiO x and SiO x @TiO 2 NPs devices, Fig. 6 shows the stochastic circuit breaker (CB) simulation that was conducted. The simulation method was benchmarked to previous research, as reported by Brivio et al. 38 In the SiO x resistive switching device, both the experimental and simulated results show good agreement, which is also exhibited by the bipolar resistive switching behavior. The relative relations between parameters for SiO 2 and TiO 2 in CBs were determined based on DFT calculation. Table 1 tabulates the parameters. Figure 6b shows that the resistances of CBs are initialized with two values of R high,S and R low,S to simulate the insulating oxide and conducting oxide layer, such as SiO 2 and oxygen-deficient SiO 2-x , respectively. To emulate the oxygen-deficient SiO x matrix, most of the CBs are initialized with R high,S , while the rest of the CBs are initialized with R low,S in the initial state. In this case, the ratio of R high,S :R low,S is set to approximately 6:4, and the switching probabilities of R high,S and R low,S depend on the electric field and temperature by Joule heating. On applying the negative volage, a few of the CBs are changed from R high,S to R low,S in sequence from #1 to #2 in Fig. 6b. Then, the voltage is applied above the , almost all CBs abruptly transform to R low,S , and the device achieves the SET process, as shown in #3 of Fig. 6b. In contrast, when the positive voltage is swept to the RESET voltage, R high,S is continuously increased, then the CB network finally reaches the RESET process (according to the blue arrows in sequence from #3 to #5 in Fig. 6b). This cycle of SET/RESET is reversibly obtained on sweeping the voltage. Likewise, the experimental I-V curve of the SiO x @TiO 2 NPs device agrees well with the simulated result, as shown in Fig. 6c. In Fig. 6 d, the maps of CB network are composed with four values of CBs of R low,S , R high,S , R low,T , and R high,T , which indicate SiO 2-x , SiO 2 , TiO 2-x , and TiO 2 , respectively. Initially, almost all CBs with R high,S and R low,S are randomly allocated in places. Based on the XPS analysis, the ratio of oxygen deficient R low,S is equalized to that of the SiO x device. The R high,T and R low,T of TiO 2 NPs are also randomly distributed with the proportion of about 5%, to mimic the TiO 2 NPs inserted SiO x matrix. Similarly, the initial ratio of R high,S :R low,S and R high,T :R low,T is established to be about 6:4, respectively, as shown in Fig. 6d. On increasing the negative voltage, the CBs related to TiO 2 NPs are more rapidly transited from R high,T to R low,T than those of SiO 2 under the applied voltage. And, when further voltage is applied to the SiO x @TiO 2 NPs device, the CBs related to SiO x are also changed from R high,S to R low,S , and the SET process is achieved according to the red arrows (in sequence from #1 to #3) in Fig. 6d. This is related to the TiO 2 NPs assisting the construction of the conductive path in SiO x @TiO 2 NPs, and causes lower SET voltages than that of the pristine SiO x device. Under the positive voltage sweeps, the CBs of TiO 2 NPs are rapidly changed from R low,T to R high,T , while the CBs of SiO x are slightly transited. Also, the first-RESET process can be achieved in sequence from #3 to #4, as shown in Fig. 6d. On further increasing the positive voltage, the R high,S is increased, then the RS gradually reaches second-HRS (in sequence from #4 to #6 in Fig. 6d). The 2-step RESET processes can be obtained by controlling the RESET voltages. In this study, the simulation method, which is simply expressed by the formation/rupture of conductive path with the stochastic CB model in the case of oxygen-deficient oxide matrix and nanoparticle-inserted oxide resistive memory devices, enables lower computational load for each CB network simulation than the conventional simulation methods. From the CB simulation, the conductive path based on the oxygen vacancies is stochastically examined under sweeping the external voltage. The difference in the switching of the SiO x and SiO x @TiO 2 NPs devices, such as the multiple-RESET, lowering switching voltage, and increase of on/off ratio, can be obtained by inserting the TiO 2 NPs.
In our case, the SET process is abrupt by applying the negative voltages, which is related to the electric field inducing the defect migration, and then causing an increase of the current. In contrast, the gradual RESET process is due to the conductive filament being gradually ruptured when the positive voltage is applied to the devices 11,39 . Also, the improvement of the LRS/HRS ratio is noteworthy, as shown in Fig. 3a and b. This can be correlated to the environment for conductive filament growth inside the RS layer being changed due to the insertion of TiO 2 NPs 40 . By applying the positive voltage, the conductive filament can easily rupture due to the existence of TiO 2 NPs inside the SiO x matrix, and carriers have difficulty in flowing inside the RS layer. Therefore, the current level of HRS for the SiO x @TiO 2 NPs is lower than that of the SiO x device.

Conclusion
We demonstrate the improvement of the resistive switching behavior in the solution processed SiO x device by the insertion of TiO 2 NPs, which is structured with ITO/SiO x @TiO 2 NPs/ITO on glass substrate. The SiO x @TiO 2 NPs resistive switching device exhibits stable bipolar resistive switching behavior. Also, outstanding switching characteristics, such as the higher ratio of SET/RESET, lower SET/RESET voltages, improved cycle-to-cycle variability, faster switching speed, and controllable multiple-RSs (LRS, HRS1, and HRS2) by applying voltages, can be obtained, in comparison to the pristine SiO x device. Based on stochastic circuit breaker simulation, we can conclude that the enhancement in switching performance in the SiO x @TiO 2 NPs originates from the difference in the formation and rupture of conductive filament by the inserted TiO 2 NPs. Fabrication of the SiO x based devices. To fabricate the SiO x -based resistive switching memory device with cross-bar array architecture, the lift-off process was conducted on glass substrate 26 . The SiO x solution diluted with ethanol was dropped on the patterned bottom electrode (BE), and spin-coated for deposition of 50 nm thick SiO x at 5000 rpm for 60 s. In the case of SiO x @TiO 2 NPs film, SiO x solution and TiO 2 NPs dispersed solution were mixed in the ratio of 1:7, and then spin-coated at the same condition. After that, the SiO x and SiO x @TiO 2 NPs films were dried at 80 °C for 20 min in oven, and then annealed using furnace at 450 °C for 1 h. The top electrode (TE) was also formed by using the lift-off process, and the cross-bar array ReRAM architecture was finally obtained with active device of 20 µm × 20 µm. During XPS measurement, Ar ion sputtering was conducted at 500 V for 10 s to eliminate carbon contamination on the surface. To investigate the composition and chemical bonding state of the SiO x and SiO x @TiO 2 NPs, X-ray photoelectron spectroscopy (XPS, ESCA Versaprobe II) was conducted by monochromatic X-ray radiation at energy hv = 1486.7 eV (Al Kα source) with pass energy of 29.5 eV. The resistive switching behavior was observed using current-voltage (I-V), which was measured by semiconductor analyzer (Keithley-4200). To contact the bottom electrode, the upper SiO x layer was lightly scraped off using a thin tip because the bottom electrode was completely covered with SiO x layer. During I-V measurement, the voltage was applied to TE, and BE was grounded. The electronic structure of the SiO x and TiO 2 NPs inserted SiO x system, and density functional theory (DFT) calculations, were conducted with the Vienna Ab Initio Simulation Package (VASP) with MedeA GUI 43,44 . Electronic structures for the α-phase of SiO 2 and the anatase phase of TiO 2 were considered to predict the ReRAM switching in the SiO x and TiO 2 NPs inserted SiO x system. For all calculations, we used the PBEsol functional with 500 eV cut-off energy [45][46][47] . During unit cell calculation, to have the k-spacing of less than 0.2/Å, we chose a 9 × 9 × 7 and 9 × 9 × 5 grid of k-points for SiO 2 and TiO 2 , respectively. Geometric optimization was performed using an RMM-DIIS algorithm, iterated until the 0.01 eV/Å condition was satisfied. To determine the electronics structure of defects in SiO 2 and TiO 2 , we generated 2 × 2 × 1 supercell for both SiO 2 and TiO 2 . Single oxygen vacancy in various charging states (V O ++ , V O + , V O 0 , V O -, V O -) were generated in both supercell structures, and geometrically optimized until the 0.01 eV/Å condition was satisfied with a 5 × 5 × 3 grid of k-points and 5 × 5 × 3 grid of k-points for SiO 2 and TiO 2 , respectively. Calculation with hybrid function (HSE06) was performed to evaluate the accurate position of defect states for optimized structures that contained defects 48 . The formation energy of the charged defect was computed using Eq. 49 :

Analysis.
where E(q) is the total energy of the supercell with charge q, E(n) is the total energy of a neutral supercell, µ e is the chemical potential of electron (Fermi level), and ΔV is the shift of energy level of the valance band maximum. To explain the resistive switching mechanism based on formation/rupture of oxygen vacancies circuit breaker (CB) modeling was conducted using Matlab program. The CB network was connected by horizontal and vertical CB resistors (90 × 30), which were composed with two resistances of R low and R high . During the CB modeling, the voltage was applied to all nodes in the top lines, while all nodes in the bottom lines were grounded.